Mobile networks are currently evolving from pure circuit switched, CS, networks towards Internet Protocol IP based networks, and by seamlessly integrating into IP based infrastructure that is also used for the internet, the World Wide Web and the datacom industry. With this trend mobile networks closely follow the evolution steps of the wireline networks, where Voice over Internet Protocol, VoIP, via Digital Subscriber Line, DSL, access or via Wireless Local Area Network, WLAN, access is existing technology today.
Mobile operators that install IMS, IP Multimedia Subsystem, networks and offer IMS services want to make these services available to Global System for Mobile Communications/Wideband Code Division Multiple Access, GSM/WCDMA, subscribers. Currently, calls from and to this group of GSM/WCDMA subscribers are to be routed through the IMS network in order to reach the IMS service engine.
If there is no full radio coverage capable of bidirectional speech Packet Switched access, that is, the UE is not able to connect to a Packet Switched access network and thus to the IMS, then the circuit switched access must be used. The concept is called IMS Centralized Services, ICS. This service is used to carry bi-directional speech media and to access the IMS. However, users cannot, control IMS services and sessions in a suitable way over a circuit switched access.